Production of melt fused synthetic fibers using a spinneret

ABSTRACT

A process and apparatus for the production of polyacrylontrile (PAN) polymer fibers from a polyacrylonitrile polymer, wherein a polyacrylonitrile polymer that comprises 90 weight percent or more polyacrylonitrile, optionally mixed with from about 30 to about 50 weight percent, based on the weight of the polymer, of a fugitive plasticizer, is heated, provided to an extruder in liquid form, extruded to form polyacrylonitrile fibers, and the fibers, immediately after the extrusion, are cooled, preferably in an air-cooled manifold, to a temperature of from about 110 to about 135° C.  
     Fibers so produced have increased tensile strength, and thus are useful for more purposes, than typically produced polyacrylonitrile fibers.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the priority of U.S. provisional patentapplication No. 60/307,630 filed Jul. 24, 2001. The disclosure of saidprovisional patent application is incorporated herein by reference forall legal purposes capable of being served thereby.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

[0002] NOT APPLICABLE

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK.

[0003] NOT APPLICABLE

BACKGROUND OF THE INVENTION

[0004] This invention resides in the field of polymeric fibers,particularly polyacrylonitrile fibers, and processes for theirmanufacture.

[0005] The acrylic fibers industry, a 60-year old enterprise, haswitnessed little growth over the past decade, in part because suchfibers are produced from a polyacrylonitrile (PAN)/solvent solution.However, currently, hot melt heat-processable thermoplastic polymers arepreferred for many uses, particularly because they can be produced atlower cost. Despite this growth problem, the acrylic fibers industryproduces about 5.3 billion pounds of fibers annually across the globe.This is due to such fibers having desirable properties such as a closelikeness to wool in feel or “handle”, high tensile strength, longresistance to ultraviolet degradation, high temperature resistance, andresistance to most common chemicals. Even more important is the singularability of PAN fibers to be transformed by oxidation and carbonizationinto PAN carbon fiber, the relatively new high tech material that is tentimes stronger than steel, pound for pound.

[0006] Thermoplastic polymers such as polyester, nylon and polypropyleneare relatively easy to process into fibers. They therefore possess aprimary advantage over PAN fibers, even though they may not have some ofthe same uses and properties, such as transformation to carbonizedfibers. To form fibers the thermoplastic polymers are simply melted byheat into an extruder, pumped to a spinneret and spun into fibersthrough the holes in the spinneret. While still warm, the fibers can betensilized or stretched to the desired degree using a convenient speed,and are then spooled or wound up.

[0007] Non-thermoplastic polymers such as PAN and rayon cannot beconverted to fibers by simple heat processing. Instead, these polymersare dissolved in a suitable solvent and the solution is pumped to thespinneret as a syrup. The pump pressure forces the syrup through theholes in the spinneret to form the fibers.

[0008] Acrylic fiber polymer is based on non-thermoplastic acrylonitrile(AN) monomer typically comprising 85% by weight of AN monomercopolymerized with 15% by weight of “neutral” co-monomers such as ethylacrylate, methyl methacrylate, and vinyl acetate. Such co-monomersimpart sites on the fiber to which coloring agents can attach, as wellas higher tensile properties for tensilizing (stretching) the fibers asthey are formed at high speeds into continuous filaments. They alsoprovide higher tensile strength for high-speed fiber production.However, when the PAN fiber is to be used to make PAN carbon fibers, theco-monomers provide a disadvantage. Oxidation and carbonization of themrequires a slow buildup of heat, for a time of from 30-120 minutes orlonger, to prevent the fibers from overheating by internal means, with aresulting loss of cohesive property values (“runaway” heat rise).

[0009] These problems can be overcome by use of a process such asdescribed in U.S. Pat. No. 5,304,590, which is hereby incorporatedherein by reference. This patent discloses a process wherein PANpolymers having AN monomer levels of from 90 to 99.6% by weight, andcontaining no neutral co-monomers, can be melt fused in a standardplastics extruder at high-speed running rates, into a clear polymericmelt that performs as though it were a thermoplastic compound. Oneexample cited is a PAN polymer comprised of 99.5% by weight AN monomer,polymerized with an FDA approved chain extender cross-linking agentcomprising the other 0.5% by weight of the polymer. Such chain extendercross-linking agent eliminates the need for the neutral co-monomer. Theuse of an efficient molecular chain extender cross-linking agent enablessuch PAN to be made into biaxially oriented (stretched in both themachine direction [MDO] and the transverse direction [TDO]) thin filmswithout fibrillating, while imparting high flexibility to the films,without brittleness. It also provides superior mechanical, electricaland barrier properties by allowing a greater amount of AN monomer to beavailable in both films and fibers.

[0010] In addition, using polymers produced by the process of U.S. Pat.No. 5,304,590, enables reduction in the amount of fugitive plasticizerfrom 70 to only 40% by weight of the PAN polymer, which materiallyreduces the cost of manufacture of these melt fused PAN fibers while atthe same time increasing the amount of AN monomer by almost 15% byweight.

[0011] In the process of U.S. Pat. No. 5,304,590, the PAN polymer usedfor melt fusion is washed and rinsed thoroughly, with its moistureadjusted to about 0.5% by weight. The PAN is dry blended with ethylenecarbonate (EC) as a fugitive plasticizer. The mixture is fed to astandard low shear vented extruder that fuses the materials into asmooth polymeric melt at relatively low temperature for pelletizing,precursor sheet for thin biaxial film preparation, or fiber production.Production process heat volatilizes EC from the formed product, and itis ducted to a cooling tower, where the EC changes at 95° F. (35° C.)into small crystals that may be used in subsequent production runs.

[0012] However, when attempting to make acrylic fibers formed by aspinneret from such a PAN polymer, it was found that such filamentstended to break too readily under the extrusion temperatures requiredfor high-speed extrusion. This may be due to the lack of thermoplastictypes of “neutral co-monomers”.

[0013] It thus would be desirable to provide PAN fibers from PANpolymer, and particularly from PAN produced by melt fusion, in which thefibers possessed a greater tensile strength.

BRIEF SUMMARY OF THE INVENTION

[0014] In brief, the invention comprises an improvement in a process forthe production of polyacrylonitrile fibers from a polyacrylonitrilepolymer wherein a polyacrylonitrile polymer that comprises 90 weightpercent or more polyacrylonitrile, optionally mixed with from about 30to about 50 weight percent, based on the weight of the polymer, of afugitive plasticizer, is heated, provided to an extruder in liquid form,and extruded to form polyacrylonitrile fibers, the improvementcomprising,

[0015] cooling the fibers, immediately after the extrusion, to atemperature of from about 110 to about 135° C.

[0016] In another aspect the invention comprises a cooling manifoldthrough which the fibers are passed, and in which the cooling step iscarried out.

[0017] Other aspects of the invention will become apparent to thoseskilled in the art from the description that follows.

BRIEF DESCRIPTION OF THE DRAWING

[0018]FIG. 1 generally depicts a cooling manifold as used in thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] PAN polymer is a white, fine powder that is dissolved in a polarsolvent such as dimethyl formamide (DMF), dimethyl sulfoxide (DMSO) orsimilar solvents or combinations of proprietary types. Dissolution istypically 70% by weight solvent to 30% by weight PAN. This provides ahot syrup which in conventional fiber production is pumped to aspinneret (a die having a multitude of orifices through which the syrupexits the spinneret) thus forming the filaments. Generally, the diameterof such filaments ranges from 0.015″ to 0.002″. The filaments aretensilized by passing them around a series of rollers, each running at afaster rate than the one before it, thereby causing the filaments tohave their molecular chains aligned in the machine direction, whichincreases tensile properties. The degree of stretching thins thefilament to the desired diameter. The filaments are given selected typesof treatments for specific end use enhancements, spooled and sold. Thesolvent is removed from the fibers by either “dry spinning” by passingthem through a heated chamber while tensilizing, or by “wet spinning”;i.e., submerging the filaments in a solvent-water trough to coagulatethe fibrils by washing, then drying them, as they are tensilized. Thesolvent volatilization vapor is ducted to a recovery system, condensed,purified, and reused in subsequent batches.

[0020] In losing its solvent during production, the circular fiberprofile collapses into many odd profile shapes (boomerang, dog bone,etc.) giving the fibers great spring or wool-like hand. The departingsolvent also causes the formation of microscopic pores in the fiber thatallow the fiber to “breathe” or absorb moisture for wearing apparelcomfort. Thermoplastic fiber polymers do not have this valuablecharacteristic.

[0021] Melt fused PAN polymers dry blended with EC fugitive plasticizerhaving a ratio mixture of 60%-40% by weight respectively, are optimallyextruded at 160-175° C. (320-347° F.). According to this invention, ithas been found that by reducing the temperature of the filaments to110-135° C. (248-275° F.) immediately after extrusion, their tensilestrength materially increases, to provide the necessary tensile strengthto carry out the tensilizing function at high speed. A mechanical meansof reducing such filament temperature rapidly and accurately to achievehigher tensile properties, is a key element to this invention. Thepreferred temperature, regardless of fiber diameter, is 120° C.

[0022] The lower temperature level allows the amount of EC plasticizerto remain at a high level in the polymer melt allowing for easy anduniform stretching. Reinforcing this process is the contribution ofhaving a PAN polymer richer in AN monomer available to stretch manytimes its extruded length to diameter. The PAN polymer preferred has abeginning average molecular weight of 80,000, which propagates to230,000 during processing. Presently used PAN polymers typically have astarting molecular weight of 30,000, which remains at or near such levelthroughout the process, when making fiber from a solution of PAN andsolvent.

[0023] A separate high nitrile PAN polymer, especially tailored as aprecursor for the production of PAN carbon fiber, which cannot be meltfused but contains no neutral co-monomers, can also benefit from thepresent invention, as it too needs protection from too frequent fibrilbreakage for high speed tensilizing.

[0024] This invention consists of an oven into which formed fibrils fromeither the melt fusion process, or from the PAN/solvent solutionprocess, passes into a perfectly controlled environment to optimize thefibril tensile properties for downstream treatment of such fibrils.

[0025] Combined with use of the melt fusion process of U.S. Pat. No.5,304,590 for non-thermoplastic PAN polymers, this invention canmaterially reduce the cost of production of PAN-based fibers, therebypermitting PAN to better compete with thermoplastic polymers. This isespecially true for precursor PAN fibers for PAN carbon fiberproduction. PAN is the only polymer which can be oxidized and carbonizedinto carbon fibers with superior properties, to meet the challenge ofthe industry need to reduce the cost and improve the quality of such PANcarbon fibers and their anticipated high tech growth.

[0026] The temperature reduction feature of this invention causes thenewly formed fibers to reach a steady state with respect to beingnon-sticky, holding a shape and contour definition, and a greatlyenhanced tensile strength, without engaging the chain extendercross-linking agents of the polymer. At the same time, the level of ECplasticizer at such stage of production remains within a percentage ortwo of its level when exiting the spinneret, thereby capable of highperformance as a plasticizer for the tensilizing steps of production tofollow.

[0027] There are numerous design possibilities for the many spinneretstyles to be encountered in setting up this mechanism for cooling PANfibers just out of the spinneret. One suitable fiber take-off designwould have holes that match each fiber exit hole from the spinneret, toallow each fiber to pass through such holes in the form of tubes madefrom an appropriate material such as stainless steel. These tubes wouldextend from the manifold's top side adjacent to the spinneret, to thebottom side of the manifold in the interior space between the top andbottom interior surfaces. Connected to the manifold would be entry andexit ports to transport a fluid medium such as water, oil, ethyleneglycol or a combination of materials for optimum functionality. Cooledair may also be used as a coolant medium in such a manifold chillerdevice. By controlling the cooling media temperature coordinated withthe fiber diameter and rate of throughput passage of the fibers, exactmelt temperature of the PAN fiber can be attained and held.

[0028] Alternatively, the manifold may have one or more large openingsin its upper surface to receive fibers, and one or more larger interiorspaces in the manifold through which the fibers are passed while beingcooled by indirect heat exchange with the fluid.

[0029] Another suitable design would include an integrated spinneret andcooling unit rather than two separate pieces of equipment. In such adesign, fibers exiting a spinneret would pass through an integratedcooling unit, which may include tubes or larger openings for the fibersto pass through, a shell for the cooling fluid, and an exit opening oropenings for the cooled fibers. 1281 At their optimum handlingtemperature for tensilizing, the formed filaments pass from thetempering manifold into an enclosed chamber heated to that attained inthe manifold. Upon entering the heated chamber, a series of godet rollswould capture the fibers and tensilize them by having each roll rotatingat a faster rate than the preceding roll. The number of fibers per rollis a decision made by the fiber producers regularly, everywhere fibersare produced. It is common in the acrylic fiber industry for spinneretsto have 150,000 orifices or more, from which the filament formingresin/solvent solution exits.

[0030] Alternatively, the atmospherically controlled chamber into whichthe newly formed filaments would proceed from the spinneret may be asimple oven-like unit. Its climate (temperature and EC vapor content)under strict control prevents the surface of the filaments from dryingout too rapidly, and thereby allow full freedom of the EC within thefilaments to pass through to the exterior of such filaments' surface.Under such conditions, the melt flow capability of the PAN melt canreplace any minute EC exit tunnels caused by the volatilization of suchEC from the filaments.

[0031] Optimally, to reduce the fibril temperature efficiently and at alow cost, a manifold or custom-sized and matched air chamber is made andinstalled just under the spinneret, and fastened to it mechanically. Thecooling unit is leak-proof around the spinneret, but is custom matchedto the underside of the spinneret in such a way that the holes from thespinneret from which the fibrils exit it, line up with correspondingholes in the manifold and the fibers pass directly through the coolingunit. The air space inside the cooling unit may be as small as an inchor so wide from top to bottom, or may be larger, as convenient andappropriate. Preferably the manifold is attached to the spinneret sothat the direction of flow of the formed fibrils is straight downwardsrather than horizontally or upwards.

[0032] Processed clean filtered air from an outside supply source whichcan be adjusted in temperature to better comply with varying fibrilthicknesses, is introduced into the manifold at several locations inorder that the temperature be uniform throughout the manifold.

[0033]FIG. 1 is a diagram showing an appropriate cooling manifold foruse in the process of this invention. In that figure, a cooling manifold2 having holes (not shown) through which the fibers may pass is directlyand precisely attached to the outlet side of a spinneret 1 in such a waythat the holes in the manifold match those in the spinneret. Air oranother cooling medium is introduced into the manifold via inlets 3,sized and arranged to provde an even distribution of the cooling mediumwithin the manifold, and is removed via outlets (not shown) situated onthe same or on the opposite side of the manifold. Fibers 4 exit from thelower end of the manifold, ready to be tensilized, spooled, etc.downstream.

[0034] At the operating temperature of the cooler, the EC plasticizervolatilizes actively. The EC vapors also assist in keeping the exteriorsurfaces of the fibrils EC-moist, which assists in allowing freedom forEC from the core of the fibril to escape through the outside of thefibril surface by the higher pressure from the fibril inside, due toheat.

[0035] The exit of the fibrils from the cooler is through a slot acrossthe bottom surface of the cooler, so that the fibrils pass in neat orderas produced in the spinneret. The row or other configuration of fibrilscooled in such way as this is picked up by conventional means andprepared for spooling for downstream secondary operations to suit theproducer.

[0036] The filaments exit the first tensilizing chamber and enter asimilar tensilizing oven heated to 140-165° C. (284F-329° F.) forfurther tensilizing. At this temperature range, the chain extendercross-linking agent within the polymer is activated. They impart thehigher molecular weight and tensile properties discussed above, by theintertwining of the molecular chains within the polymer, as well astheir propagation and the creation of crystalline domains within thepolymer.

[0037] A third stage tempering oven would complete the total removal byvolatilization of any residual EC vapors from the tensilized PAN fibers,using temperatures to about 204° C. (400° F.).

[0038] At this point in the overall process, the filaments are ready forstandard high-speed spooling operations. Having been tensilized, thinnedto the desired diameter, free of the EC fugitive plasticizer, and withthe molecular chains reaching a propagation to 230,000, these filamentshave excellent tensile properties for further downstream secondaryoperations, including oxidization and carbonization in the production ofvery high quality PAN carbon fibers.

What is claimed is: 1 In a process for the production of polyacrylonitrile fibers from a polyacrylonitrile polymer, wherein a polyacrylonitrile polymer that comprises 90 weight percent or more polyacrylonitrile, optionally mixed with from about 30 to about 50 weight percent, based on the weight of the polymer, of a fugitive plasticizer, is heated, provided to an extruder in liquid form, and extruded to form polyacrylonitrile fibers, the improvement comprising, cooling the fibers, immediately after the extrusion, to a temperature of from about 110 to about 135° C.
 2. A process according to claim 1 wherein the polyacrylonitrile polymer contains a chain extender-cross linking agent.
 3. A process according to claim 1 wherein the fugitive plasticizer is ethylene carbonate.
 4. A process according to claim 1 wherein the cooling is conducted at a temperature of about 120° C.
 5. A process according to claim to claim 1, further comprising carbonizing the polyacrylonitrile fibers.
 6. A process according to claim 1 wherein the cooling is conducted in an air-cooled manifold that is attached to, and in flow connection with, the extruder.
 7. A process according to claim 1 in which the cooling is conducted in a manifold that is attached to, and in flow connection with, the extrude, the manifold being cooled by a liquid cooling agent selected from water, oil, and specialty coolants.
 8. A process according to claim 1 wherein the polyacrylonitrile polymer is produced by a melt fusion process.
 9. Apparatus for cooling polymeric fibers produced by extrusion, by indirect heat exchange with a cooling medium, comprising: (a) a plurality of fiber inlet openings sized and arranged so as to match openings through which the fibers are extruded; (b) a plurality of tubes connected to the plurality of fiber inlet openings, sized for passage of fibers therethrough; (c) a shell surrounding the plurality of tubes; (d) inlet and outlet means in the shell, for introducing thereto and removing therefrom a fluid heat exchange medium, whereby the fluid heat exchange medium is passed in indirect heat exchange with fibers passing through the plurality of tubes; and (e) outlet means for removing fibers from the apparatus.
 10. Integrated apparatus for extruding fibers and for cooling the extruded fibers, comprising (a) an extruding section wherein fibers are extruded from a liquid polymeric material introduced thereinto; (b) a cooling section located directly below the extruding section for receiving and cooling the extruded fibers; (c) inlet means in the cooling section for introducing extruded fibers thereinto; (d) passage means for passing the fibers through the cooling section; (e) a shell surrounding the passage means; (f) inlet and outlet means in the shell, for introducing thereto and removing therefrom a fluid heat exchange medium, whereby the fluid heat exchange medium is passed in indirect heat exchange with fibers passing through the cooling section; and (g) outlet means for removing the fibers from the cooling section. 